Intersystem crossing agents for efficient utilization of excitons in organic light emitting devices

ABSTRACT

Organic light emitting devices are described wherein the emissive layer comprises a host material containing a fluorescent or phosphorescent emissive molecule, which molecule is adapted to luminesce when a voltage is applied across the heterostructure, wherein an intersystem crossing molecule of optical absorption spectrum matched to the emission spectrum of the emissive molecule enhances emission efficiency.

I. FIELD OF INVENTION

The present invention is directed to organic light emitting devices(OLEDs) comprised of emissive layers that contain an organic compoundfunctioning as an emitter and a separate intersystem crossing (“ISC”)entity which operates to enhance the efficiency of the emission.

II. BACKGROUND OF THE INVENTION II. A. General Background

Organic light emitting devices (OLEDs) are comprised of several organiclayers in which one of the layers is comprised of an organic materialthat can be made to electroluminesce by applying a voltage across thedevice, C. W. Tang et al., Appl. Phys. Lett. 1987, 51, 913. CertainOLEDs have been shown to have sufficient brightness, range of color andoperating lifetimes for use as a practical alternative technology toLCD-based full color flat-panel displays (S. R. Forrest, P. E. Burrowsand M. E. Thompson, Laser Focus World, Feb. 1995). Since many of thethin organic films used in such devices are transparent in the visiblespectral region, they allow for the realization of a completely new typeof display pixel in which red (R), green (G), and blue (B) emittingOLEDs are placed in a vertically stacked geometry to provide a simplefabrication process, a small R-G-B pixel size, and a large fill factor,International Patent Application No. PCT/US95/15790.

A transparent OLED (TOLED), which represents a significant step towardrealizing high resolution, independently addressable stacked R-G-Bpixels, was reported in International Patent Application No.PCT/US97/02681 in which the TOLED had greater than 71% transparency whenturned off and emitted light from both top and bottom device surfaceswith high efficiency (approaching 1% quantum efficiency) when the devicewas turned on. The TOLED used transparent indium tin oxide (ITO) as thehole-injecting electrode and a Mg—Ag-ITO electrode layer forelectron-injection. A device was disclosed in which the ITO side of theMg—Ag-ITO layer was used as a hole-injecting contact for a second,different color-emitting OLED stacked on top of the TOLED. Each layer inthe stacked OLED (SOLED) was independently addressable and emitted itsown characteristic color. This colored emission could be transmittedthrough the adjacently stacked, transparent, independently addressable,organic layer or layers, the transparent contacts and the glasssubstrate, thus allowing the device to emit any color that could beproduced by varying the relative output of the red and bluecolor-emitting layers.

PCT/US95/15790 application disclosed an integrated SOLED for which bothintensity and color could be independently varied and controlled withexternal power supplies in a color tunable display device. ThePCT/US95/15790 application, thus, illustrates a principle for achievingintegrated, full color pixels that provide high image resolution, whichis made possible by the compact pixel size. Furthermore, relatively lowcost fabrication techniques, as compared with prior art methods, may beutilized for making such devices.

II.B. Background of emission

II.B.1. Basics

II.B.1.a. Singlet and Triplet Excitons

Because light is generated in organic materials from the decay ofmolecular excited states or excitons, understanding their properties andinteractions is crucial to the design of efficient light emittingdevices currently of significant interest due to their potential uses indisplays, lasers, and other illumination applications. For example, ifthe symmetry of an exciton is different from that of the ground state,then the radiative relaxation of the exciton is disallowed andluminescence will be slow and inefficient. Because the ground state isusually anti-symmetric under exchange of spins of electrons comprisingthe exciton, the decay of a symmetric exciton breaks symmetry. Suchexcitons are known as triplets, the term reflecting the degeneracy ofthe state. For every three triplet excitons that are formed byelectrical excitation in an OLED, only one symmetric state (or singlet)exciton is created. (M. A. Baldo, D. F. O'Brien, M. E. Thompson and S.R. Forrest, Very high-efficiency green organic light-emitting devicesbased on electrophosphorescence, Applied Physics Letters, 1999, 75,4-6.) Luminescence from a symmetry-disallowed process is known asphosphorescence. Characteristically, phosphorescence may persist for upto several seconds afier excitation due to the low probability of thetransition. In contrast, fluorescence originates in the rapid decay of asinglet exciton. Since this process occurs between states of likesymmetry, it may be very efficient.

Many organic materials exhibit fluorescence from singlet excitons.However, only a very few have been identified which are also capable ofefficient room temperature phosphorescence from triplets. Thus, in mostfluorescent dyes, the energy contained in the triplet states is wasted.However, if the triplet excited state is perturbed, for example, throughspin-orbit coupling (typically introduced by the presence of a heavymetal atom), then efficient phosphoresence is more likely. In this case,the triplet exciton assumes some singlet character and it has a higherprobability of radiative decay to the ground state. Indeed,phosphorescent dyes with these properties have demonstrated highefficiency electroluminescence.

Only a few organic materials have been identified which show efficientroom temperature phosphorescence from triplets. In contrast, manyfluorescent dyes are known (C. H. Chen, J. Shi, and C. W. Tang, “Recentdevelopments in molecular organic electroluminescent materials,”Macromolecular Symposia, 1997, 125, 1-48; U. Brackmann, LambdachromeLaser Dyes (Lambda Physik, Gottingen, 1997) and fluorescent efficienciesin solution approaching 100% are not uncommon. (C. H. Chen, 1997, op.cit.) Fluorescence is also not affected by triplet-triplet annihilation,which degrades phosphorescent emission at high excitation densities. (M.A. Baldo, et al., “High efficiency phosphorescent emission from organicelectroluminescent devices,” Nature, 1998, 395, 151-154; M. A. Baldo, M.E. Thompson, and S. R. Forrest, “An analytic model of triplet-tripletannihilation in electrophosphorescent devices,” 1999). Consequently,fluorescent materials are suited to many electroluminescentapplications, particularly passive matrix displays.

II.B.1.b. Overview of invention relative to basics

This invention pertains to the use of intersystem crossing agents toenhance emission efficiency in organic light emitting devices. Anintersystem crossing agent, or molecule, is one which can undergointersystem crossing, which involves the transfer of population betweenstates of different spin multiplicity. Lists of known intersystemcrossing agents, or molecules, are given in A. Gilbert and J. Baggott,Essentials of Molecular Photochemistry, Blackwells Scientific, 1991.

In one embodiment of the present invention, we focus on a way to use anintersystem crossing agent to increase efficiency in a system with afluorescent emitter. Therein, we describe a technique whereby tripletsformed in the host material are not wasted, but instead are transferredto the singlet excited state of a fluorescent dye. In this way, allexcited states are employed and the overall efficiency of fluorescenceincreased by a factor of four. In this embodiment, the ISC agent trapsthe energy of excitons and transfers the energy to the fluorescentemitter by a Forster energy transfer. The energy transfer processdesired is:

³D*+¹A→¹D+¹A*  (Eq. 1)

Here, D and A represent a donor molecule and a fluorescent acceptor,respectively. The superscripts 3 and 1 denote the triplet and singletstates, respectively, and the asterisk indicates the excited state.

In a second embodiment of the present invention, we focus on a way touse an intersystem crossing agent to increase efficiency in a systemwith a phosphorescent emitter. Therein, we describe a technique wherebythe ISC agent is responsible for converting all of the excitons from ahost material into their triplet states and then transferring thatexcited state to the phosphorescent emitter. This would include the casewherein the ISC agent only traps singlet excitons on the host and hosttriplet excitons are transferred directly to the phosphorescent emitter(rather than going through the ISC agent.)

In this second embodiment wherein phosphorescent efficiency is enhanced,a phosphorescent emitter is combined with an intersystem crossing agentsuch that the following can occur:

 ¹D*+¹X→¹D+¹X*

¹X*→³X*

³X*+¹A→¹X+³A*

³A*→¹A+hν

wherein D represents the donor (host), X represents the intersystemcrossing agent, and A represents the acceptor (emissive molecule).Superscript 1 denotes singlet spin multiplicity; superscript 3 denotestriplet spin multiplicity and the asterisk denotes an excited state.

In a third embodiment of the present invention, we focus on a way to usean intersystem crossing agent to increase efficiency by acting as afilter and a converter. In one aspect of the filter/converterembodiment, the intersystem crossing agent acts to convert singletexcitons to triplet excitons, thereby keeping singlets from reaching theemissive region and thus enhancing optical purity (the “filter” aspect:singlets are removed and thus no singlets emit) and increasingefficiency (the “conversion” aspect: singlets are converting totriplets, which do emit).

These embodiments are discussed in more detail in the examples below.However the embodiments may operate by different mechanisms. Withoutlimiting the scope of the invention, we discuss the differentmechanisms.

II.B.1.c. Dexter and Förster mechanisms

To understand the different embodiments of this invention it is usefulto discuss the underlying mechanistic theory of energy transfer. Thereare two mechanisms commonly discussed for the transfer of energy to anacceptor molecule. In the first mechanism of Dexter transport (D. L.Dexter, “A theory of sensitized luminescence in solids,” J. Chem. Phys.,1953, 21, 836-850), the exciton may hop directly from one molecule tothe next. This is a short-range process dependent on the overlap ofmolecular orbitals of neighboring molecules. It also preserves thesymmetry of the donor and acceptor pair (E. Wigner and E. W. Wittmer,Uber die Struktur der zweiatomigen Molekelspektren nach derQuantenmechanik, Zeitschrift fur Physik, 1928, 51, 859-886; M.Klessinger and J. Michl, Excited states and photochemistry of organicmolecules (VCH Publishers, New York, 1995). Thus, the energy transfer ofEq. (1) is not possible via Dexter mechanism. In the second mechanism ofFörster transfer (T. Förster, Zwischenmolekulare Energiewanderung andFluoreszenz, Annalen der Physik, 1948, 2, 55-75; T. Förster, Fluoreszenzorganischer Verbindugen (Vandenhoek and Ruprecht, Gottinghen, 1951), theenergy transfer of Eq. (1) is possible. In Förster transfer, similar toa transmitter and an antenna, dipoles on the donor and acceptormolecules couple and energy may be transferred. Dipoles are generatedfrom allowed transitions in both donor and acceptor molecules. Thistypically restricts the Förster mechanism to transfers between singletstates.

However, in one embodiment of the present invention, we consider thecase where the transition on the donor (³D*→¹D) is allowed, i.e. thedonor is a phosphorescent molecule. As discussed earlier, theprobability of this transition is low because of symmetry differencesbetween the excited triplet and ground state singlet.

Nevertheless, as long as the phosphor can emit light due to someperturbation of the state such as due to spin-orbit coupling introducedby a heavy metal atom, it may participate as the donor in Förstertransfer. The efficiency of the process is deternined by the luminescentefficiency of the phosphor (F Wilkinson, in Advances in Photochemistry(eds. W. A. Noyes, G. Hammond, and J. N. Pitts, pp. 241-268, John Wiley& Sons, New York, 1964), i.e. if a radiative transition is more probablethan a non-radiative decay, then energy transfer will be efficient. Suchtriplet-singlet transfers were predicted by Förster (T. Förster,“Transfer mechanisms of electronic excitation,” Discussions of theFaraday Society, 1959, 27, 7-17) and confirmed by Ermolaev andSveshnikova (V. L. Ermolaev and E. B. Sveshnikova, “Inductive-resonancetransfer of energy from aromatic molecules in the triplet state,”Doklady Akademii Nauk SSSR, 1963, 149, 1295-1298), who detected theenergy transfer using a range of phosphorescent donors and fluorescentacceptors in rigid media at 77K or 90K. Large transfer distances areobserved, for example, with triphenylamine as the donor and chrysoidineas the acceptor, the interaction range is 52 Å.

The remaining condition for Forster transfer is that the absorptionspectrum should overlap the emission spectrum of the donor assuming theenergy levels between the excited and ground state molecular pair are inresonance. In Example 1 of this application, we use the green phosphorfac tris(2-phenylpyridine) iridium (Ir(ppy)₃; M. A. Baldo, et al., Appl.Phys. Lett., 1999, 75, 4-6) and the red fluorescent dye[2-methyl-6-[2-(2,3,6,7-tetrahydro-1H,5H-benzo[ij]quinolizin-9-yl)ethenyl]-4H-pyran-ylidene]propane-dinitrile] (“DCM2”; C. W. Tang, S. A. VanSlyke, and C. H. Chen,“Electroluminescence of doped organic films,” J. Appl. Phys., 1989, 65,3610-3616). DCM2 absorbs in the green, and, depending on the localpolarization field (V. Bulovic, et al., “Bright, saturated,red-to-yellow organic light-emitting devices based onpolarization-induced spectral shifts,” Chem. Phys. Lett., 1998, 287,455-460), it emits at wavelengths between λ=570 nm and λ=650 nm.

It is possible to implement Forster energy transfer from a triplet stateby doping a fluorescent guest into a phosphorescent host material.Unfortunately, such systems are affected by competitive energy transfermechanisms that degrade the overall efficiency. In particular, the closeproximity of the host and guest increase the likelihood of Dextertransfer between the host to the guest triplets. Once excitons reach theguest triplet state, they are effectively lost since these fluorescentdyes typically exhibit extremely inefficient phosphorescence.

Another approach is to dope both the phosphorescent donor and thefluorescent acceptor into a host material. The energy can then cascadefrom the host, through the phosphor sensitizing molecule and into thefluorescent dye following the equations (collectively Eq. 2):

³D*+¹X→¹D+³X*

³X*+¹A→¹X+¹A*

¹A*→¹A+hν  (2a)

 ¹D*+¹X→¹D+¹X*

¹X*→³X*

³X*+¹A→¹X+¹A*

¹A*→¹A+hν  (2b)

wherein X represents the sensitizer molecule and hν is the photonenergy.

The multiple state energy transfer required in thephosphorescent-sensitized system is schematically described in FIG. 1.Dexter transfers are indicated by dotted arrows, and Forster transfersby solid arrows. Transfers resulting in a loss in efficiency are markedwith a cross. In addition to the energy transfer paths shown in thefigure, direct electron-hole recombination is possible on thephosphorescent and fluorescent dopants as well as the host. Tripletexciton formation after charge recombination on the fluorescent dye isanother potential loss mechanism.

To maximize the transfer of host triplets to fluorescent dye singlets,it is desirable to maximize Dexter transfer into the triplet state ofthe phosphor while also minimizing transfer into the triplet state ofthe fluorescent dye. Since the Dexter mechanism transfers energy betweenneighboring molecules, reducing the concentration of the fluorescent dyedecreases the probability of triplet-triplet transfer to the dye. On theother hand, long range Förster transfer to the singlet state isunaffected. In contrast, transfer into the triplet state of the phosphoris necessary to harness host triplets, and may be improved by increasingthe concentration of the phosphor. To demonstrate the multiple statetransfer, we used 4,4′-N,N′-dicarbazole-biphenyl (“CBP”) as the host (D.F. O'Brien, M. A. Baldo, M. E. Thompson, and S. R. Forrest, “Improvedenergy transfer in electrophosphorescent devices,” Appl. Phys. Lett.,1999, 74, 442-444), Ir(ppy)₃ as the phosphorescent sensitizer, and DCM2as the fluorescent dye. The doping concentration was 10% for Ir(ppy)₃,and 1% for DCM2.

The details, given in Example 1 below, showed the improvement inefficiency of fluorescent yield brought about by the use of thephosphorescent sensitizer. In the following sections, we give additionalbackground.

II.B.2. Interrelation of Device Structure and Emission

Devices whose structure is based upon the use of layers of organicoptoelectronic materials generally rely on a common mechanism leading tooptical emission. Typically, this mechanism is based upon the radiativerecombination of a trapped charge. Specifically, OLEDs are comprised ofat least two thin organic layers separating the anode and cathode of thedevice. The material of one of these layers is specifically chosen basedon the material's ability to transport holes, a “hole transportinglayer” (HTL), and the material of the other layer is specificallyselected according to its ability to transport electrons, an “electrontransporting layer” (ETL). With such a construction, the device can beviewed as a diode with a forward bias when the potential applied to theanode is higher than the potential applied to the cathode. Under thesebias conditions, the anode injects holes (positive charge carriers) intothe hole transporting layer, while the cathode injects electrons intothe electron transporting layer. The portion of the luminescent mediumadjacent to the anode thus forms a hole injecting and transporting zonewhile the portion of the luminescent medium adjacent to the cathodeforms an electron injecting and transporting zone. The injected holesand electrons each migrate toward the oppositely charged electrode. Whenan electron and hole localize on the same molecule, a Frenkel exciton isformed. Recombination of this short-lived state may be visualized as anelectron dropping from its conduction potential to a valence band, withrelaxation occurring, under certain conditions, preferentially via aphotoemissive mechanism. Under this view of the mechanism of operationof typical thin-layer organic devices, the electroluminescent layercomprises a luminescence zone receiving mobile charge carriers(electrons and holes) from each electrode.

As noted above, light emission from OLEDs is typically via fluorescenceor phosphorescence. There are issues with the use of phosphorescence. Ithas been noted that phosphorescent efficiency can decrease rapidly athigh current densities. It may be that long phosphorescent lifetimescause saturation of emissive sites, and triplet-triplet annihilation mayalso produce efficiency losses. Another difference between fluorescenceand phosphorescence is that energy transfer of triplets from aconductive host to a luminescent guest molecule is typically slower thanthat of singlets; the long range dipole-dipole coupling (Förstertransfer) which dominates energy transfer of singlets is (theoretically)forbidden for triplets by the principle of spin symmetry conservation.Thus, for triplets, energy transfer typically occurs by diffusion ofexcitons to neighboring molecules (Dexter transfer); significant overlapof donor and acceptor excitonic wavefunctions is critical to energytransfer. Another issue is that triplet diffusion lengths are typicallylong (e.g., >1400 Å) compared with typical singlet diffusion lengths ofabout 200 Å. Thus, if phosphorescent devices are to achieve theirpotential, device structures need to be optimized for tripletproperties.

In this invention, we exploit the property of long triplet diffusionlengths to improve external quantum efficiency.

Successful utilization of phosphorescence holds enormous promise fororganic electroluminescent devices. For example, an advantage ofphosphorescence is that all excitons (formed by the recombination ofholes and electrons in an EL), which are (in part) triplet-based inphosphorescent devices, may participate in energy transfer andluminescence in certain electroluminescent materials. In contrast, onlya small percentage of excitons in fluorescent devices, which aresinglet-based, result in fluorescent luminescence.

An alternative is to use phosphorescence processes to improve theefficiency of fluorescence processes. Fluorescence is in principle 75%less efficient due the three times higher number of symmetric excitedstates. In one embodiment of the present invention, we overcome theproblem by using a phosphorescent sensitizer molecule to excite afluorescent material in a red-emitting OLED. The mechanism for energeticcoupling between molecular species is a long-range, non-radiative energytransfer from the phosphor to the fluorescent dye. Using this technique,the internal efficiency of fluorescence can be as high as 100%, a resultpreviously only possible with phosphorescence. As shown in Example 1, weemploy it to nearly quadruple the efficiency of a fluorescent OLED.

II.C. Background of Materials

II.C.1. Basic Heterostructures

Because one typically has at least one electron transporting layer andat least one hole transporting layer, one has layers of differentmaterials, forming a heterostructure. The materials that produce theelectroluminescent emission may be the same materials that functioneither as the electron transporting layer or as the hole transportinglayer. Such devices in which the electron transporting layer or the holetransporting layer also functions as the emissive layer are referred toas having a single heterostructure. Alternatively, theelectroluminescent material may be present in a separate emissive layerbetween the hole transporting layer and the electron transporting layerin what is referred to as a double heterostructure. The separateemissive layer may contain the emissive molecule doped into a host orthe emissive layer may consist essentially of the emissive molecule.

That is, in addition to emissive materials that are present as thepredominant component in the charge carrier layer, that is, either inthe hole transporting layer or in the electron transporting layer, andthat function both as the charge carrier material as well as theemissive material, the emissive material may be present in relativelylow concentrations as a dopant in the charge carrier layer. Whenever adopant is present, the predominant material in the charge carrier layermay be referred to as a host compound or as a receiving compound.Materials that are present as host and dopant are selected so as to havea high level of energy transfer from the host to the dopant material. Inaddition, these materials need to be capable of producing acceptableelectrical properties for the OLED. Furthermore, such host and dopantmaterials are preferably capable of being incorporated into the OLEDusing materials that can be readily incorporated into the OLED by usingconvenient fabrication techniques, in particular, by usingvacuum-deposition techniques.

II.C.2. Exciton Blocking Layer

The exciton blocking layer used in the devices of the present invention(and previously disclosed in U.S. appl. Ser. No. 09/154,044)substantially blocks the diffusion of excitons, thus substantiallykeeping the excitons within the emission layer to enhance deviceefficiency. The material of blocking layer of the present invention ischaracterized by an energy difference (“band gap”) between its lowestunoccupied molecular orbital (LUMO) and its highest occupied molecularorbital (HOMO) In accordance with the present invention, this band gapsubstantially prevents the diffusion of excitons through the blockinglayer, yet has only a minimal effect on the turn-on voltage of acompleted electroluminescent device. The band gap is thus preferablygreater than the energy level of excitons produced in an emission layer,such that such excitons are not able to exist in the blocking layer.Specifically, the band gap of the blocking layer is at least as great asthe difference in energy between the triplet state and the ground stateof the host.

For a situation with a blocking layer between a hole-conducting host andthe electron transporting layer (as is the case in Example 1, below),one seeks the following characteristics, which are listed in order ofrelative importance.

1. The difference in energy between the LUMO and HOMO of the blockinglayer is greater than the difference in energy between the triplet andground state singlet of the host material.

2. Triplets in the host material are not quenched by the blocking layer.

3. The ionization potential (IP) of the blocking layer is greater thanthe ionization potential of the host. (Meaning that holes are held inthe host.)

4. The energy level of the LUMO of the blocking layer and the energylevel of the LUMO of the host are sufficiently close in energy such thatthere is less than 50% change in the overall conductivity of the device.

5. The blocking layer is as thin as possible subject to having athickness of the layer that is sufficient to effectively block thetransport of excitons from the emissive layer into the adjacent layer.

That is, to block excitons and holes, the ionization potential of theblocking layer should be greater than that of the HTL, while theelectron affinity of the blocking layer should be approximately equal tothat of the ETL to allow for facile transport of electrons.

[For a situation in which the emissive (“emitting”) molecule is usedwithout a hole transporting host, the above rules for selection of theblocking layer are modified by replacement of the word “host” by“emitting molecule.”]

For the complementary situation with a blocking layer between aelectron-conducting host and the hole-transporting layer one seekscharacteristics (listed in order of importance):

1. The difference in energy between the LUMO and HOMO of the blockinglayer is greater than the difference in energy between the triplet andground state singlet of the host material.

2. Triplets in the host material are not quenched by the blocking layer.

3. The energy of the LUMO of the blocking layer is greater than theenergy of the LUMO of the (electron-transporting) host. (Meaning thatelectrons are held in the host.)

4. The ionization potential of the blocking layer and the ionizationpotential of the host are such that holes are readily injected from theblocker into the host and there is less than a 50% change in the overallconductivity of the device.

5. The blocking layer is as thin as possible subject to having athickness of the layer that is sufficient to effectively block thetransport of excitons from the emissive layer into the adjacent layer.

[For a situation in which the emissive (“emitting”) molecule is usedwithout an electron transporting host, the above rules for selection ofthe blocking layer are modified by replacement of the word “host” by“emitting molecule.”]

II.D. Color

As to colors, it is desirable for OLEDs to be fabricated using materialsthat provide electroluminescent emission in a relatively narrow bandcentered near selected spectral regions, which correspond to one of thethree primary colors, red, green and blue so that they may be used as acolored layer in an OLED or SOLED. It is also desirable that suchcompounds be capable of being readily deposited as a thin layer usingvacuum deposition techniques so that they may be readily incorporatedinto an OLED that is prepared entirely from vacuum-deposited organicmaterials.

U.S. Pat. No. 08/774,333, filed Dec. 23, 1996, is directed to OLEDscontaining emitting compounds that produce a saturated red emission.

III. SUMMARY OF THE INVENTION

At the most general level, the present invention is directed to organiclight emitting devices comprising an emissive layer wherein the emissivelayer comprises an emissive molecule, with a host material (wherein theemissive molecule present as a dopant in said host material) whichmolecule is adapted to luminesce when a voltage is applied across aheterostructure, wherein the emissive molecule is selected from thegroup of phosphorescent or fluorescent organic molecules and wherein thedevice comprises a molecule which can function as an intersystemcrossing agent (“ISC molecule”) which improves the efficiency of thephosphorescence or fluorescence relative to the situation where the ISCmolecule is absent. It is preferred that the emissive molecule and theintersystem crossing molecule be different and it is preferred thatthere be substantial spectral overlap between the emissive molecule andthe intersystem crossing molecule.

In a first embodiment wherein fluorescent efficiency is enhanced, afluorescent emitter is combined with a phosphorescent sensitizer, whichoperates as an intersystem crossing agent. The phosphorescent sensitizermay be selected from materials wherein the radiative recombination rateis much greater than the non-radiative rate of recombination. Thephosphorescent sensitizer may be selected from the group ofcyclometallated organometallic compounds. The metal thereof may beselected from metals of the third row of the periodic table (especiallyW, Pt, Au, Ir, Os) and any other metals or metal compounds that havestrong spin orbit coupling. The phosphorescent sensitizer may be furtherselected from the group of phosphorescent organometallic iridium orosmium complexes and may be still further selected from the group ofphosphorescent cyclometallated iridium or osmium complexes. A specificexample of the sensitizer molecule is fac tris(2-phenylpyridine)iridium, denoted (Ir(ppy)₃) of formula

[In this, and later figures herein, we depict the dative bond fromnitrogen to metal (here, Ir) as a straight line.]

A specific example of the fluorescent emitter is DCM2, of formula

In a second embodiment of the present invention, wherein phosphorescentefficiency is enhanced, a phosphorescent emitter is combined with anintersystem crossing agent such that the following can occur:

¹D*+¹X→¹D+¹X*

¹X*→³X*

³X*+¹A→¹X+³A*

³A*→¹A+hν

wherein D represents the donor (host), X represents the intersystemcrossing agent, and A represents the acceptor (emissive molecule).Superscript 1 denotes singlet spin multiplicity; superscript 3 denotestriplet spin multiplicity and the asterisk denotes an excited state.

In a third embodiment of the present invention, a thin layer of an ISCagent is placed in the device; it may be between the HTL and ETL. TheISC agent is selected such that the optical absorption spectrum of theISC agent overlaps strongly with the emission line of the material foundat the site of recombination.

The general arrangement of the heterostructure of the devices is suchthat the layers are ordered hole transporting layer, emissive layer, andelectron transporting layer. For a hole conducting emissive layer, onemay have an exciton blocking layer between the emissive layer and theelectron transporting layer. For an electron conducting emissive layer,one may have an exciton blocking layer between the emissive layer andthe hole transporting layer. The emissive layer may be equal to the holetransporting layer (in which case the exciton blocking layer is near orat the anode) or to the electron transporting layer (in which case theexciton blocking layer is near or at the cathode).

The emissive layer may be formed with a host material in which theemissive molecule resides as a guest. The host material may be ahole-transporting matrix selected from the group of substituted tri-arylamines. An example of a host material is 4,4′-N,N′-dicarbazole-biphenyl(CBP), which has the formula

The emissive layer may also contain a polarization molecule, present asa dopant in said host material and having a dipole moment, that affectsthe wavelength of light emitted when said emissive dopant moleculeluminesces.

A layer formed of an electron transporting material is used to transportelectrons into the emissive layer comprising the emissive molecule andthe optional host material. The electron transport material may be anelectron-transporting matrix selected from the group of metalquinoxolates, oxidazoles and triazoles. An example of an electrontransport material is tris-(8-hydroxyquinoline) aluminum (Alq₃).

A layer formed of a hole transporting material is used to transportholes into the emissive layer comprising the emissive molecule and theoptional host material. An example of a hole transporting material is4,4′-bis[N-(1-naphthyl)-N-phenyl-amino] biphenyl [“a-NPD”].

The use of an exciton blocking layer (“barrier layer”) to confineexcitons within the luminescent layer (“luminescent zone”) is greatlypreferred. For a hole-transporting host, the blocking layer may beplaced between the luminescent layer and the electron transport layer.An example of a material for such a barrier layer is2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (also called bathocuproineor BCP), which has the formula

IV. BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1. Proposed energy transfer mechanisms in the multi-step systemIdeally, all excitons are transferred to the singlet state of thefluorescent dye, as triplets in the dye non-radiatively recombine.Forster transfers are represented by solid lines and Dexter transfers bydotted lines. Electron-hole recombination creates singlet and tripletexcitons in the host material. These excitons are then transferred tothe phosphorescent sensitizer. There is also a lower probability ofdirect transfer to the fluorescent dye by Förster transfer into thesinglet state, or Dexter transfer into the triplet state. This lattermechanism is a source of loss and this is signified in the figure by across. Singlet excitons in the phosphor are then subject to intersystemcrossing (ISC) and transfer to the triplet state. From this state, thetriplets may either dipole-dipole couple with the singlet state of thefluorescent dye or in another loss mechanism, they may Dexter transferto the triplet state. Note also that electron-hole recombination is alsopossible on the phosphor and fluorescent dye. Direct formation oftriplets on the fluorescent dye is an additional loss. Inset. Thestructure of electroluminescent devices fabricated in this work. Themultiple doped layers are an approximation to a mixed layer of CBP: 10%Ir(ppy)₃: 1% DCM2. Two variants were also made. Second device: TheIr(ppy)₃ was exchanged with Alq₃ to examine the case where theintermediate step is fluorescent and not phosphorescent. Third device:Separately, a device containing a luminescent layer of CBP: 1% DCM2 wasmade to examine direct transfers between CBP and DCM2.

FIG. 2. The external quantum efficiencies of DCM2 emission in the threedevices. The sensitizing action of Ir(ppy)₃ clearly improves theefficiency. Note also the presence of Alq₃ in the all-fluorescentdevices makes little or no difference.

FIG. 3. In the spectra of the three devices, characteristic peaks areobserved for CBP (λ˜400 nm), TPD (λ˜420 nm), Alq₃ (λ˜490 nm) Ir(ppy)₃(λ˜400 nm) and DCM2 (λ˜590 nm). Approximately 80% of the photons in theIr(ppy)₃ device are emitted by DCM2. All spectra were recorded at acurrent density of ˜1 mA/cm².

FIG. 4. The transient response of the DCM2 and Ir(ppy)₃ components inthe CBP: 10% Ir(ppy)₃: 1% DCM2 device. The transient lifetime of DCM2 is˜1 ns, thus in the case of energy transfer from Ir(ppy)₃, the responseof DCM2 should be governed by the transient lifetime of Ir(ppy)₃. Afterthe initial 100 ns-wide electrical excitation pulse, this is clearly thecase, demonstrating that energy is transferred from the triplet state inIr(ppy)₃ to the singlet state in DCM2. However, during the excitationpulse, singlet transfer to DCM2 is observed, resulting in the ripples inthe transient response. These ripples are due to fluctuations in thecurrent density and the discharge of traps at the falling edge of thepulse. Note that the trends in the DCM2 and Ir(ppy)₃ transient responseeventually diverge slightly. This is due to a small amount of chargetrapped on DCM2 molecules recombining and causing luminescence.

FIG. 5. Schematic of device containing a layer of ISC agent between ETLand HTL.

FIG. 6. IV characteristics for the device described in Example 5/FIG. 5.

V. DETAILED DESCRIPTION OF THE INVENTION

The present invention is directed to organic light emitting devices(OLEDs) comprised of emissive layers that contain an organic compoundfunctioning as an emitter and a separate intersystem crossing (“ISC”)molecule which operates to enhance the efficiency of the emission.Embodiments are described which enhance emission efficiency forfluorescent emitters and for phosphorescent emitters.

It is preferred that there be substantial spectral overlap between theISC molecule and the emissive molecule. One way of measuring spectraloverlap is by integrating absorption and emission spectra over the rangeof energies (wavenumbers) over which both spectra have non-zero values.This approach is related to that taken in Equation 2(a) of A.Shoustikov, Y. You, and M. E. Thompson, “Electroluminescence ColorTuning by Dye Doping in Organic Light Emitting Diodes,” IEEE Journal ofSpecial Topics in Quantum Electronics, 1998, 4, 3-14. One approach is tonormalize the absorption and emission spectra to integrated intensitiesof one. One integrates the product of the normalized spectra over therange of energies where both spectra have non-zero values. This rangemay be taken to be 180 nm to 1.5 microns in wavelength. If the value isat least 0.01, and more preferably at least 0.05, one has substantialspectral overlap.

It is also preferred that there be substantial spectral overlap betweenthe emission spectrum of the host material and the absorption spectrumof the ISC agent. One integrates the product of the normalized spectraover the range of energies where both spectra have non-zero values. Thisrange may be taken to be 180 nm to 1.5 microns in wavelength. If thevalue is at least 0.01, and more preferably at least 0.05, one hassubstantial spectral overlap.

The present invention will now be described in detail for specificpreferred embodiments of the invention, it being understood that theseembodiments are intended only as illustrative examples and the inventionis not to be limited thereto.

V.A. Use of ISC Agents to Enhance Fluorescent Emission

V.A.1. Overview of First Embodiment

An embodiment of the present invention is generally directed tophosphorescent sensitizers for fluorescent emissive molecules, whichluminesce when a voltage is applied across a heterostructure of anorganic light-emitting device and which sensitizers are selected fromthe group of phosphorescent organometallic complexes, and to structures,and correlative molecules of the structures, that optimize the emissionof the light-emitting device. The term “organometallic” is as generallyunderstood by one of ordinary skill, as given, for example, in“Inorganic Chemistry” (2nd edition) by Gary L. Miessler and Donald A.Tarr, Prentice-Hall (1998). The invention is further directed tosensitizers within the emissive layer of an organic light-emittingdevice which molecules are comprised of phosphorescent cyclometallatediridium complexes. Discussions of the appearance of color, includingdescriptions of CIE charts, may be found in Color Chemistry, VCHPublishers, 1991 and H. J. A. Dartnall, J. K. Bowmaker, and J. D.Mollon, Proc. Roy. Soc. B (London), 1983, 220, 115-130.

V.A.2. Examples of First Embodiment

The structure of the organic devices of Examples 1, 2, and 3 is shown inthe inset of FIG. 1.

EXAMPLE 1

Organic layers were deposited by a high vacuum (10⁻⁶ Torr) thermalevaporation onto a clean glass substrate pre-coated with a 1400 Å-thicklayer of transparent and conductive indium tin oxide. A 600 Å-thicklayer ofN,N′-diphenyl-N,N′-bis(3-methylphenyl)-[1,1′-biphenyl]-4,4′-diamine[“TPD”] is used to transport holes to the luminescent layer. Theluminescent layer consisted of an alternating series of 10 Å-thicklayers of CBP doped to 10% (by mass) of Ir(ppy)₃, and 10 Å thick layersof CBP doped to 1% (by mass) of DCM2. In total, 10 doped layers weregrown, with a total thickness of 100 Å. Excitons were confined withinthe luminescent region by a 200 Å-thick layer of the exciton-blockingmaterial 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (also calledbathcuproine, or BCP). A 300 Å-thick layer of the electron transportmaterial tris-(8-hydroxyquinoline) aluminum (“Alq₃”) is used totransport electrons to the luminescent region and to reduce absorptionat the cathode. A shadow mask with 1 mm-diameter openings was used todefine the cathodes consisting of a 1000 Å-thick layer of 25:1 Mg:Ag,with a 500 Å-thick cap. The compound Ir(ppy)₃ [sensitizer/ISC agent inExample 1] has the following formulaic representation:

COMPARATIVE EXAMPLE 2

As one control, a device was created as in Example 1, except thatIr(ppy)₃ was replaced by Alq₃, which has similar emission and absorptionspectra, but no observable phosphorescence at room temperature.

COMPARATIVE EXAMPLE 3

As a second control, a device was created as in Example 1, except thatthe intermediate energy transfer step was omitted to examine directenergy transfer from CBP to DCM2.

Results of Examples 1, 2, and 3

The external quantum efficiency (photons per electron) as a function ofinjection current of the DCM2 portion of the emission spectrum for eachexample is given in FIG. 2. The DCM2 emission efficiency of the devicecontaining the phosphorescent sensitizer is significantly higher thanits fluorescent analog. Indeed, the peak efficiency of (3.3±0.1) %,significantly higher than the best result of ˜2% observed for DCM2 inprevious studies (C. H. Chen, C. W. Tang, J. Shi, and K. P. Klubeck,“Improved red dopants for organic luminescent devices,” MacromolecularSymposia, 1997, 125, 49-58)). This demonstrates that host triplets aretransferred to the fluorescent singlet state in Example 1. As a morequantitative comparison of the increase in emission due to thesensitizer, we note the difference in the quantum efficiency of DCM2emission, where the maximum efficiency is 0.9±0.1% in the examplewithout the phosphorescent sensitizer and 3.3% in the example withphosphorescent sensitizer [Refer to FIG. 2 and the addition of Alq₃ tothe CBP:DCM2 device in Comparative Example 2.] The ratio of efficiencyof sensitized to unsensitized devices is 3.7±0.4, which is close to thevalue of four (4) expected between emission of (singlet+triplet) to(only singlet) [that is, (1+3)/(1+0)] for devices in which theprobability of both singlet and triplet participation is equal.

The emission spectra of the OLEDs of the three examples are given inFIG. 3. All devices show energy transfer to the fluorescent dye. Bytaking the area under the various spectral peaks, we find thatapproximately 80% of photons are emitted by DCM2 in the devicecontaining the Ir(ppy)₃ sensitizer. The remainder contribute to CBPluminescence at λ˜400 nm, TPD luminescence at λ˜420 nm, and Ir(ppy)₃luminescence at λ˜500 nm. In the device doped with 10% Alq₃, an emissionpeak is observed at λ˜490 nm. This is consistent with observations ofAlq₃ emission in a non-polar host (CBP). (V. Bulovic, R. Deshpande, M.E. Thompson, and S. R. Forrest, “Tuning the color emission of thin filmmolecular organic light emitting devices by the solid state solvationeffect,” Chemical Physics Letters (1999).

Conclusive evidence of the energy transfer process in Eq. 2 is shown inFIG. 4, which illustrate the transient behavior of the DCM2 and lr(ppy)₃components of the emission spectra. These data were obtained by applyinga 100 ns electrical pulse to the electroluminscent device The resultingemission was measured with a streak camera. If a fraction of the DCM2emission originates via transfer from Ir(ppy)₃ triplets (Eq. 2), thenthe proposed energy transfer must yield delayed DCM2 fluorescence.Furthermore, since the radiative lifetime of DCM2 is much shorter thanthat of ir(ppy)₃, the transient decay of DCM2 should match that oflr(ppy)₃. After an initial peak, most probably due to singlet-singlettransfer (Eq. 2), the DCM2 decay does indeed follow the Ir(ppy)₃ decay.The transient lifetime of Ir(ppy)₃ in this system is ˜100 ns, comparedto a lifetime of ˜500 ns in the absence of DCM2, confirming an energytransfer of ˜80%. The decrease in the triplet lifetime as a result ofenergy transfer to the fluorescent acceptor is advantageous. Not onlydoes it increase the transient response of the system but also it hasbeen shown that the probability of triplet-triplet annihilation variesinversely with the square of the triplet lifetime. (M. A. Baldo, M. E.Thompson, and S. R. Forrest, “An analytic model of triplet-tripletannihilation in electrophosphorescent devices,” (1999).) Thus, it isexpected that this multi-stage energy transfer will reduce the quenchingof triplet states, thereby further enhancing the potential for higherefficiency sensitized fluorescence.

The three examples demonstrate a general technique for improving theefficiency of fluorescence in guest-host organic systems. Furtherimprovement may be expected by mixing the host, phosphorescentsensitizer, and fluorescent dye rather than doping in thin layers as inthis work, although the thin layer approach inhibits direct Dextertransfer of triplets from the host to the fluorophore where they wouldbe lost. To reduce losses further in the multi-state energy transfer, anideal system may incorporate low concentrations of a sterically hindereddye. For example, adding spacer groups to the DCM2 molecule shoulddecrease the probability of Dexter transfer to the dye while minimallyaffecting its participation in Forster transfer or its luminescenceefficiency. Since Dexter transfer can be understood as the simultaneoustransfer of an electron and a hole, steric hindrance may also reduce thelikelihood of charge trapping on the fluorescent dye. Similar effortshave already reduced non-radiative excimer formation in a DCM2 variant[Chen, Tang, Shi and Klubeck, “Improved red dopants for organic ELDevices, Macromolecular Symposia, 1997, 125, 49-58]. Also, optimizationof the device structure will reduce Ir(ppy)₃ emission to lower levels.

V.B. Use of ISC Agents to Enhance Phosphorescent Emission

V.B.1. Overview of Second Embodiment

The second embodiment is directed to the situation wherein the emissivemolecule is phosphorescent and the use of intersystem crossing moleculesenhances the efficiency of the phosphorescent emission.

V.B.2. Example of Second Embodiment

Prophetic Example 4.

An OLED is fabricated with a traditional diamine hole transporter and anelectron transporting layer (ETL) composed of three different materials.The ETL is roughly 80% a traditional electron transporting material(such as Zrq4), 15% an intersystem crossing agent (such as benzil; otherISC agents may be found in the reference of Gilbert and Baggott) and 5%a phosphorescent emitter (such as PtOEP, platinum octaethyl porphyrin).The ISC agent is chosen so that its absorption spectrum overlapsstrongly with the ETL's fluorescence spectrum. Hole electronrecombination occurs at or near the HTL/ETL interface generating amixture of singlet and triplet excitons. The singlet excitons on the ETLwill efficiently transfer their energy to the ISC agent, which willefficiently intersystem cross to its triplet state, via a n→π* state orsome other suitable process.

The triplet energy of the ISC agent will then transfer to the dopant andemission will occur at the phosphorescent dopant. Triplet excitonsformed on the ETL will either transfer directly to the dopant or energytransfer to the ISC agent, which will transfer that energy to the dopantas described. The ISC agent in this application is designed tocompletely quench singlet excitons giving a good yield of tripletexcitons for transfer to the phosphorescent dopant.

The chemical formula of Zrq₄ is

V.C. Use of Intersystem Crossing Agent as Filter and Converter

V.C.1. Overview of Third Embodiment

In this third embodiment of the present invention, a thin layer of anISC agent is placed between the HTL and ETL. The ISC agent is selectedsuch that the optical absorption spectrum of the ISC agent overlapsstrongly with the emission line of the material found at the site ofrecombination.

In the control experiments discussed below, we utilized 2,7 diphenylfluorenone (“ISC-F”) as the ISC agent. An ISC agent suitable for thefilter/converter embodiment can be selected from the group consisting ofacridines, acridones, brominated polycyclic aromatic compounds,anthraquinones, alpha-beta-diketones, phenazines, benzoquinones,biacetyls, fullerenes, thiophenes, pyrazines, quinoxalines, andthianthrenes.

V.C.2. Examples of Third Embodiment

EXAMPLE 5

In FIGS. 5 and 6, we present control experiments for a device without aphosphorescent dopant emitter. An example of the third embodiment canhave a phosphorescent emitter in the ETL layer.

The structure of the device for this example is given schematically inFIG. 5. It is made of a heterostructure with α-NPD/ISC-F/Alq3. (The Alq3layer is not doped). The IV characteristic of the device is given inFIG. 6. The device area here is 3.14 mm². The key point is that there isno light at low to medium bias. This result shows that the ISCfilter/converter certainly quenches singlets. [At very high biases (>17Volts) weak green emission can be observed. The spectrum of this outputshows that it is from Alq3. To explain the emission, either there areelectrons leaking through to Alq3 at high bias or the ISC-F istransferring energy back to the singlet in Alq3.]

In the device corresponding to the third embodiment of the presentinvention, the Alq3 region is doped with a phosphorescent emitter. Wewould know that triplet excitons have been efficiently injected into theAlq3 layer because of phosphorescent emission arising from the dopedemitter.

In the embodiment of the invention contemplated, the 2,7-diphenylfluorenone (“ISC-F”) transports electrons to the α-NPD/ISC-F interface.Hole/electron recombination at or near this interface leads to bothsinglet and triplet excitons. Both of these excitons will be readilytransferred to the ISC-F layer. Any singlet that transfers to the ISC-Flayer (or is formed in it) will rapidly intersystem cross to a triplet.Thus, all of the excitons present will be efficiently converted totriplets within the device.

Specifically, the triplet excitons will diffuse through the ISC-F layerand transfer to the Alq₃ layer. The transfer to Alq₃ should be facile.Although the triplet energy of Alq₃ is not exactly known, it is believedto be between 550 and 600 nm. This is exactly in the correct region toefficiently trap triplet excitons from ISC-F. Using the ISC agent inthis way we prevent singlet excitons from ever reaching the emissiveregion of the device. By doping the emissive region with aphosphorescent dye, we an efficiently extract the energy luminescently.The ISC agent here is acting as a filter which only allows tripletexcitons to be injected into the Alq₃ layer. The requirements for suchan ISC filter/converter are that it have both singlet and tripletenergies below that of the material that is at or near the site ofrecombination (α-NPD in the example) and a triplet energy higher thanthe emissive region (which must not be the site of recombination, Alq₃in the example). The material must have a high ISC efficiency.

V.D. Other Discussion

V.D.1. Spectral Overlap

In the embodiments of the present invention, there should be spectraloverlap between the emissive molecule and the intersystem crossingmolecule. The nature of the overlap may depend upon the use of thedevice, which uses include a larger display, a vehicle, a computer, atelevision, a printer, a large area wall, theater or stadium screen, abillboard and a sign. For display applications of the device of thepresent invention, there should be spectral overlap in the visiblespectrum. For other applications, such as the use of this device inprinting, the overlap of the emission with the human photopic responsemay not be required.

V.D.2. Other Examples of Sensitizer/ISC Agent for First Embodiment

The embodiment of the present invention for enhancing fluorescentemission is not limited to the sensitizer molecule of the examples. Onecontemplates the use of metal complexes wherein there is sufficient spinorbit coupling to make the radiative relaxation an allowed process. Ofligands, one of ordinary skill may modify the organic component of theIr(ppy)₃ (directly below) to obtain desirable properties.

One may have alkyl substituents or alteration of the atoms of thearomatic structure.

These molecules, related to Ir(ppy)₃, can be formed from commerciallyavailable ligands. The R groups can be alkyl or aryl and are preferablyin the 3, 4, 7 and/or 8 positions on the ligand (for steric reasons).

Other possible sensitizers are illustrated below.

This molecule is expected to have a blue-shifted emission compared toIr(ppy)₃. R and R′ can independently be alkyl or aryl.

Organometallic compounds of osmium may be used in this invention.Examples are the following.

These osmium complexes will be octahedral with 6d electrons(isoelectronic with the Ir analogs) and may have good intersystemcrossing efficiency. R and R′ are independently selected from the groupconsisting of alkyl and aryl. They are believed to be unreported in theliterature.

Herein, X can be selected from the group consisting of N or P. R and R′are independently selected from the group alkyl and aryl.

V.D.3. Other Molecular Depictions

A molecule for the hole-transporting layer of the invention is depictedbelow.

The invention will work with other hole-transporting molecules known byone of ordinary skill to work in hole transporting layers of OLEDs.

A molecule used as the host in the emissive layer of the invention isdepicted below.

The invention will work with other molecules known by one of ordinaryskill to work as hosts of emissive layers of OLEDs. For example, thehost material could be a hole-transporting matrix and could be selectedfrom the group consisting of substituted tri-aryl amines andpolyvinylcarbazoles.

The molecule used as the exciton blocking layer of Example 1 is depictedbelow. The invention will work with other molecules used for the excitonblocking layer, provided they meet the requirements given herein.

V.D.4. Uses of Device

The OLED of the present invention may be used in substantially any typeof device which is comprised of an OLED, for example, in OLEDs that areincorporated into a larger display, a vehicle, a computer, a television,a printer, a large area wall, theater or stadium screen, a billboard ora sign.

The present invention as disclosed herein may be used in conjunctionwith co-pending applications: “High Reliability, High Efficiency,Integratable Organic Light Emitting Devices and Methods of ProducingSame”, Ser. No. 08/774,119 (filed Dec. 23, 1996); “Novel Materials forMulticolor Light Emitting Diodes”, Ser. No. 08/850,264 (filed May 2,1997); “Electron Transporting and Light Emitting Layers Based on OrganicFree Radicals”, Ser. No. 08/774,120 (filed Dec. 23, 1996); “MulticolorDisplay Devices”, Ser. No. 08/772,333 (filed Dec. 23, 1996);“Red-Emitting Organic Light Emitting Devices (OLED's)”, Ser. No.08/774,087 (filed Dec. 23, 1996); “Driving Circuit For Stacked OrganicLight Emitting Devices”, Ser. No. 08/792,050 (filed Feb. 3, 1997); “HighEfficiency Organic Light Emitting Device Structures”, Ser. No.08/772,332 (filed Dec. 23, 1996); “Vacuum Deposited, Non-PolymericFlexible Organic Light Emitting Devices”, Ser. No. 08/789,319 (filedJan. 23, 1997); “Displays Having Mesa Pixel Configuration”, Ser. No.08/794,595 (filed Feb. 3, 1997); “Stacked Organic Light EmittingDevices”, Ser. No. 08/792,046 (filed Feb. 3, 1997); “High ContrastTransparent Organic Light Emitting Devices”, Ser. No. 08/792,046 (filedFeb. 3, 1997); “High Contrast Transparent Organic Light Emitting DeviceDisplay”, Ser. No. 08/821,380 (filed Mar. 20, 1997); “Organic LightEmitting Devices Containing A Metal Complex of 5-Hydroxy-Quinoxaline asA Host Material”, Ser. No. 08/838,099 (filed Apr. 15, 1997); “LightEmitting Devices Having High Brightness”, Ser. No. 08/844,353 (filedApr. 18, 1997); “Organic Semiconductor Laser”, Ser. No. 08/859,468(filed May 19, 1997); “Saturated Full Color Stacked Organic LightEmitting Devices”, Ser. No. 08/858,994 (filed on May 20, 1997); “PlasmaTreatment of Conductive Layers”, PCT/US97/10252, (filed Jun. 12, 1997);“Novel Materials for Multicolor Light Emitting Diodes”, Ser. No.08/814,976, (filed Mar. 11, 1997); “Novel Materials for Multicolor LightEmitting Diodes”, Ser. No. 08/771,815, (filed Dec. 23, 1996);“Patterning of Thin Films for the Fabrication of Organic Multi-colorDisplays”, PCT/US97/10289, (filed Jun. 12, 1997), and “DoubleHeterostructure Infrared and Vertical Cavity Surface Emitting OrganicLasers”, (filed Jul. 18, 1997), each co-pending application beingincorporated herein by reference in its entirety.

What is claimed:
 1. An organic light emitting device comprising aheterostructure for producing luminescence, comprising an emissivelayer, wherein the emissive layer is a combination of a host materialand an emissive molecule, present as a dopant in said host material;wherein the emissive molecule is adapted to luminesce when a voltage isapplied across the heterostructure; and wherein the heterostructurecomprises an intersystem crossing molecule such that the efficiency ofthe emission is enhanced by the use of the intersystem crossingmolecule.
 2. The organic light emitting device of claim 1 wherein theemission spectrum of the intersystem crossing molecule substantiallyoverlaps the absorption spectrum of the emissive molecule.
 3. An organiclight emitting device comprising a heterostructure for producingluminescence, comprising an emissive layer, wherein the emissive layeris a combination of a conductive host material and a fluorescentemissive molecule, present as a dopant in said host material; whereinthe emissive molecule is adapted to luminesce when a voltage is appliedacross the heterostructure; and wherein the heterostructure comprises anintersystem crossing molecule which is an efficient phosphor whoseemission spectrum substantially overlaps with the absorption spectrum ofthe emissive molecule.
 4. The emissive layer of claim 3, wherein theintersystem crossing molecule is Ir(ppy)₃.
 5. The device of claim 3wherein the fluorescent emissive molecule is DCM2.
 6. The device ofclaim 1 wherein the emissive molecule is phosphorescent.
 7. The deviceof claim 2 wherein the emissive molecule is phosphorescent.
 8. Thedevice of claim 6 wherein the emissive molecule is PtOEP.
 9. The deviceof claim 1 wherein there is a thin layer comprising intersystem crossingagent, wherein the thin layer is placed between a hole transport layerand an electron transport layer.
 10. The device of claim 2 wherein theemissive molecule is phosphorescent and wherein a thin layer comprisingintersystem crossing agent inhibits emission from singlet states. 11.The device of claim 2 wherein the emissive molecule is phosphorescentand wherein a thin layer comprising intersystem crossing agent enhancestriplet emission.
 12. An organic light emitting device comprising aheterostructure for producing luminescence, comprising: an emissionlayer comprising a host material; and an emissive molecule, present as adopant in said host material, adapted to luminesce when a voltage isapplied across the heterostructure; a hole transport layer; an electrontransport layer; and an intersystem crossing agent wherein there issubstantial spectral overlap between the emission spectrum of theintersystem crossing agent and the absorption spectrum of the emissivemolecule.
 13. The device of claim 12 wherein emission efficiencyincreases relative to a device with the emissive molecule but withoutthe intersystem crossing agent.
 14. The device of claim 12 with anexciton blocking layer.
 15. The device of claim 14 wherein the excitonblocking layer comprises 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline.16. The device of claim 12 wherein the emissive molecule is fluorescentand the intersystem crossing agent is an organometallic compoundcomprising a metal of the third row of the periodic table.
 17. Thedevice of claim 12 wherein the emissive molecule is phosphorescent. 18.The device of claim 12 wherein the emissive molecule is phosphorescentand the intersystem crossing agent inhibits emission from singlets. 19.The organic light emitting device of claim 1 used in a member of thegroup consisting of a larger display, a vehicle, a computer, atelevision, a printer, a large area wall, theater or stadium screen, abillboard and a sign.